The invention relates to a method of inserting a drainage wick into the ground and to a device for anchoring the drainage wick.
Drainage wicks are commonly used for carrying out vertical draining in the ground in order to improve the strength of ground and generally consist of paper material in the form of strips or of a band-shaped plastic core enclosed in a suitable filter material.
Such drainage wicks are widely used to consolidate soft compressible soils (such as clays and/or fine grained soils). Consolidation of a compressible soil occurs as pore water is squeezed from the soil matrix. The installation of vertical drains provides shortened drainage paths for the water to exit the soil and thus can reduce significantly the soil settlement time.
In order to insert such a drainage wick in the ground, for example to a depth of 10 m up to 40 or 60 m, the drainage wick is threaded through an inserting tube or column after the tube is driven into the ground. When such an inserting tube is then extracted from the ground, the drainage wick remains in the soil, the soil pressurizes against the drainage wick, and water may travel through the permeable filter material of the drainage wick along the plastic core.
Such a drainage wick inserting method is disclosed in U.S. Pat. No. 2,577,252 and devices for inserting a drainage wick are disclosed in U.S. Pat. No. 3,891,186 or U.S. Pat. No. 4,755,080.
According to one prior art method, the drainage wick is threaded through the inserting tube and an anchorage component, such as a plate or bar, is attached to the lower end of said drainage wick. The drainage wick and anchorage component are then driven downwardly into the ground with or through said inserting tube. Said inserting tube is then pulled up whereby said anchorage component and said drainage wick are permitted to remain in the ground.
It has to be noted that while such a process is widely and commonly used, drawbacks still exist; namely, when the drainage wick needs to be inserted deeply and/or in soft soils. Frequently, in such conditions, the anchorage component does not stay at its bottom position when the inserting tube is pulled up from the soil and may also be pulled up, even to several meters. The efficiency of the drainage wick is then considerably reduced and in extreme cases annihilated if it is pulled out of the soil layer to be drained.
Solutions have been adopted to overcome such drawbacks such as:                driving the drainage wick and the anchorage component deeper downwardly to a hard soil layer. The anchorage component is then more easily retained in the hard soil layer and the drainage wick is maintained in its bottom position. Such a solution still presents several drawbacks. First it increases the costs, due mainly to deeper installation and slowing down the wick drainage installation process. Second, such a process may be unusable due to geological constraint, such as aquifer presence in the hard soil. It is then necessary to avoid communication between the different soil layers to prevent water contamination. Third, the process is to be avoided when using vacuum soil consolidation as disclosed, for example, in U.S. Pat. No. 6,254,308. That is, as suction means are used to evacuate the liquid collected from the ground, the draining process would be endless if drainage wicks were to be in contact with underlying permeable soils.        enlarging the anchorage component which is commonly roughly of the same size as the size of the section of the inserting tube. It is possible to use anchorage components which are significantly larger than the section of the inserting tube. They would be harder to move when the inserting tube is pulled up and remain easily in the bottom position. However, the use of such large anchorage components increases cost of such a component and significantly increases penetration resistance of the soil when installing the drainage wick.        introducing water in the inserting tube to balance hydrostatic pressure at the bottom of the inserting tube and then pushing the anchorage component in the soil, allowing it to remain in the bottom position. However, such water filling of the inserting tube is time and energy consuming and is generally to be avoided.        